In a spread spectrum method, an original signal is spread by a spread code at the transmitting end to generate a spread signal, and the spread signal is despread by the spread code at the receiving end to recover the original signal. The receiving end needs to synchronize with, i.e., acquire the spread code coming from the transmitting end in order to despread the spread signal. Achieving this synchronization is generally called “code acquisition”.
It has been known that the code acquisition is typically performed by using a sliding correlator or a matched filter.
Other methods for code acquisition have been disclosed, for example, in JP-A-H5-219011 and JP-A-H8-237170. In the method disclosed in JP-A-H5-219011, the number of matches between a received signal and a spread code are counted to detect a synchronization point, and it is determined that the synchronization point is detected when the counted number of matches exceeds a predetermined threshold. Therefore, the speed of the code acquisition can be enhanced. In the method disclosed in JP-A-H8-237170, a non-spread preamble is inserted between bits of a spread transmitted signal, and the receiving end detects a synchronization point based on the non-spread preamble.
In the method using the sliding correlator, a multiply accumulate operation between one period of a received signal from a certain start point and one period of a spread code is performed to detect a synchronization point. This operation is repeated by shifting the start point until the synchronization point is detected.
Since there is a possibility that the operation is repeated a number of times until the synchronization point is detected, the code acquisition may take much time to be completed. As a result, system responsibility may be worsened.
In the method using the matched filter, multiply accumulate operations between multiple periods having shifted start points and the spread code are performed in parallel to detect the synchronization point. Therefore, as compared to in the method using the sliding correlator, the code acquisition can be completed in a short time.
However, using a matched filter corresponds to using multiple sliding correlators that are arranged in parallel. In the method using the matched filter therefore, cost, circuit size, and power consumption may be increased as compared to in the method using the sliding correlator. Further, although the method using the matched filter enhances the speed of the code acquisition by measuring correlation (self-correlation) between the received signal and the spread code in parallel, the number of times the correlation is measured is the same between the method using the sliding correlator and the method using the matched filter.
In the method disclosed in JP-A-H5-219011, the transmitting end is required to transmit only fixed value data until the code acquisition is completed. Further, since there is a need to inform completion of the code acquisition of the transmitting end, additional time is required before data is received after the completion of the code acquisition.
In the method disclosed in JP-A-H8-237170, transmission time is increased by the preamble. Therefore, transmission speed is worsened by the preamble at the time of normal transmission and reception after the code acquisition is completed.
In summary, although the methods disclosed in JP-A-H5-219011 and JP-A-H8-237170 may simplify code acquisition, the methods may require additional structure to the transmitting end and may cause a reduction in data transmission speed after code acquisition.